This invention relates to a process for improving the adhesion of isocyanate-crosslinked polyester coatings on vulcanized articles made from ethylene-.alpha.-olefin-unconjugated diene terpolymers (EPDM).
In the metal working industry, predominantly in the automobile industry, increasing use is made -- inter alia for reasons of safety -- of coated elastomer components, e.g., bumpers, facings, decorative moldings, radiator grilles and fenders. Due to advantageous properties such as high temperature stability and high elasticity at relatively low temperatures, vulcanized articles made from ethylene-.alpha.-olefin-unconjugated diene terpolymers have proven to be particularly expedient elastomers for this utilization. However, these vulcanizates have the drawback that they normally cannot be provided with a firmly adhering coating without a pretreatment of the surface or admixture of suitable components before the molding and vulcanizing steps. Additionally, such utilization requires that the coating adhere firmly to the vulcanizate substrate even if the coated component is elastically deformed at low temperatures, e.g., about -29.degree. C.
Many attempts have heretofore been made to improve adhesion to the surface of vulcanized articles, primarily of ethylene-propylene-diene elastomers, with respect to coatings and/or to enhance the adhesive power of the coatings. However, the presently available prior-art processes all have greater or lesser disadvantages.
Thus, in the process described in DOS (German Unexamined Laid-Open Application) Nos. 2,403,663, the vulcanized elastomer components are readied for coating by adding a halogenated elastomer to the elastomer to be vulcanized. In accordance with the procedure of DOS No. 2,364,494, vulcanizates amenable to coating are obtained by adding a phenol-aldehyde resin to the elastomer. An inherent disadvantage of these methods is that the component improving the adhesion between the substrate surface and the coating is distributed over the entire article to be coated, thereby making the elastomer relatively expensive.
It is furthermore known to improve the adhesion of vulcanized articles made from EPDM elastomer by treatment with ozone, e.g., see DOS No. 2,402,214; peroxydisulfate heavy metal salts, e.g., see DAS (German Published Application) No. 2,022,918; or chromosulfuric acid, e.g., see DOS No. 2,241,413. However, the coatings of the thus-pretreated shaped articles do not meet all of the necessary requirements. Besides, these processes have the additional drawback of requiring the handling of aggressive chemicals, thereby resulting also in waste air and wastewater disposal problems.
The process which seems to be most frequently employed at present is described in DAS No. 2,063,259; SAE.Meet.Automot. Eng., Jan. 1973:1-12 and contains the following process steps: The molded articles to be coated are washed with detergents, repeatedly rinsed with water, dried, sprayed with an organic solution of benzophenone and then vigorously dried. The thusprepared articles are subsequently heated by means of infrared lamps to a temperature above the melting point of benzophenone and the heated article is irradiated with ultraviolet light. After irradiation, the articles are ready to be coated. Optionally, the irradiation period can be shortened if a highly concentrated isocyanate solution is additionally applied.
This process has the disadvantages of employing relatively a large number of process steps, of liberating organic solvents during the procedure, and of being utilizable only with difficulties or not at all in case of components having a complicated configuration with corners and cavities, due to the linear light propagation.
Therefore, there is a genuine need in the art for methods to reduce and/or eliminate the disadvantages of such prior-art processes.